Comparative study of element mercury removal by three bio-chars from various solid wastes

•Combined physical and chemical modification was used to modify the bio-chars.•Hg0 removal by chemical modification was 2–3 times of that by physical modification.•The mechanism of chemisorptions of Hg0 by bio-chars was investigated.•C6WN5 showed to be a cost-effective Hg0 capture sorbent.

Three kinds of bio-chars derived from various solid wastes, such as municipal solid wastes, agricultural wastes and medicinal residues, were modified by physical and/or chemical modification. The Hg0 adsorption capacities of the modification bio-chars in flue gas were comparatively studied. Physical modification increased the surface areas of the bio-chars, which was favorable in the physisorption of Hg0. Chemical modification resulted in the chemisorption of Hg0. Both physisorption and chemisorptions played a role in Hg0 capture, whereas chemisorption was more important. The X-ray photoelectron spectroscopy (XPS) analysis results suggested that chemisorption of Hg0 was due to the C–Cl groups generating on the surfaces of bio-chars during NH4Cl impregnation, which could transform Hg0 into HgCl2 or other Hg–Cl complexes. The bio-chars modified by combined physical and chemical modification all had an excellent performance for Hg0 removal, especially for C6WN5 exhibiting higher Hg0 adsorption than modified activated carbons or modified activated carbon fibers. It indicated that the sequence for Hg0 removal capacity decreased as: C6WN5 (from agricultural wastes, 11,400 μg/g) > MW6N5 (from medicinal residues, 840 μg/g) > W6WN5 (from municipal solid wastes, 160 μg/g).

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